Determining the age of ancient buildings is straightforward when they contain timbers, which can be readily dated by examining patterns of tree rings or by conventional radiocarbon analysis. But when no original wood is present, the situation is more challenging. One promising technique is to apply radiocarbon dating to ancient mortars, which absorb carbon dioxide from the atmosphere as they set. The difficulty is that such mortars often contain bits of limestone that were not converted to quicklime during initial preparation, or they include limestone fragments used as "aggregate." Fortunately, the effects of such contaminants can be reduced using appropriate mechanical, chemical and optical methods.